Final answer:
A voltmeter will read the system voltage when properly connected across a closed switch in a circuit with electrical power on.
Step-by-step explanation:
A voltmeter is an instrument used to measure the potential difference between two points in an electric circuit. When properly connected across a closed switch in a circuit with electrical power on, the voltmeter will read the system voltage, which is the potential difference between the two terminals of the power source. It provides an accurate measurement of the voltage across the switch without affecting the circuit appreciably.